Date   : Sun, 03 May 2009 18:55:36 +0100
From   : info@... (Sprow)
Subject: ADFS 1.53 patch

In article <zL5PGVCxdd$JFwRP@...>,
   Mike Tomlinson <mike@...> wrote:
> In article <5055be80e6info@...>, Sprow <info@...> writes
>
> >The ATAPI commands need a 16 bit interface, so even if you throw away half
> >the drive capacity you can't even issue the command without a 16 bit
> >interface.
>
> Many thanks.  I've just had a browse of JGH's site.  The separate
> references to 8-bit and 16-bit IDE interfaces have gone, and there is
> only mention of the one interface now, which appears to be the 8-bit.

Shame, since the 8 bit interface is unbuffered so can't be used with other
1MHz bus kit.

> I'm puzzled.  How does HDInit get the drive geometry from an 8-bit
> interface?

Because there's a certain amount of redundancy in the numbers, say a 512MB
drive

  0x20000000 as seen by 16 bit
  0x20xx00xx as seen by 8 bit

clearly the last byte must be zero because of the sector size, and with a
few more heuristics you could guess the other byte I suspect (and so on for
other common drive sizes above this),
Sprow.